Add allocs to benchmark instructions

When using `RenderValuesHook` we need to save the raw value, not the
rendered form.

Two aspects were not spelled out explicitly, which sometimes led to
misunderstandings:

- error messages are always printed
- the error instance is optional

funcr: Fix bug in repeated calls to WithValues

docs: clarify Error semantic

funcr: fix example verbosity

Add a new Option `MaxLogDepth` which  tells funcr how many levels of
nested fields (e.g. a struct that contains a struct that contains a
struct, etc.) it may log.  Every time it finds a struct, slice, array,
or map the depth is increased by one.  When the maximum is reached, the
value will be converted to a string indicating that the max depth has
been exceeded.  If this field is not specified, a default value will be
used.

funcr: Prevent stack overflow on recursive structs